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What
you need to know about calcium
Harvard Health Letter, April
2003
Calcium is billed as the
bone-building nutrient. But some experts
argue that we should pay more attention to
exercise and vitamin D.
Starting on your 51st birthday,
current government guidelines say you’re
supposed to consume 1,200 milligrams (mg) of
calcium daily. At about that age, both men
and women begin to experience osteoporosis, a
decline in the density of bones that makes
them weaker and more likely to break. In essence,
your bone becomes more porous, and calcium
supposedly fills in the holes.
But calcium is currently
at the center of one of nutrition’s most
contentious debates. The critics say there’s
little evidence that high intake has more than
a marginal effect on bone density and fracture
prevention. They say exercise and vitamin D
are neglected and more important for bone health.
They also argue that dairy foods are a bad
overall influence on many people’s diets
because of the saturated fat, calories, and
other factors. Professor Walter Willett, chair
of the nutrition department at the Harvard
School of Public Health and a member of the Health
Letter’s editorial board, is one
of the leading lights in the critical camp.
Naturally, the proponents
see the evidence quite differently — and
they set the government recommendations, so
they’re hardly a fringe group. They say
dozens of studies have shown that high calcium
intake builds up bone and prevents fractures.
And they cite calcium’s other possible
benefits, such as modest protection against
colon cancer.
So what should you do? For
women, 1,200–1,500 mg of calcium daily
doesn’t seem to have any drawbacks. For
men, though, it may. Studies have shown a possible
connection between calcium and prostate cancer.
Great Britain set its daily calcium recommendation
at 700 mg, which Professor Willett believes
is probably closer to the amount that men should
be consuming.
Perhaps the most important
thing you can do is change how you think about
calcium. High intake is not the surefire ticket
to bone health that it has been made out to
be.
If you’re still worried
about getting enough calcium, supplements are
inexpensive and free of the saturated fat and
added calories that you’d get from dairy
products. If you’re going to consume
dairy products, pick low-fat and skim varieties.
Food
sources
Most of the calcium in the
American diet comes from dairy products. Not
only do they contain a lot of the mineral,
but it’s in a form that’s easy
to digest and absorb. An 8-ounce serving of
plain yogurt provides about 400 mg of calcium;
an 8-ounce glass of milk, 300 mg; and a slice
of cheddar cheese, 200 mg.
Vegetables are another food
source, although figuring out how much calcium
you’re actually getting is tricky. If
a vegetable contains oxalic or phytic acid,
then the calcium may be poorly absorbed because
of the acids. For example, a cup of frozen
spinach contains almost as much calcium as
a cup of milk, but only a tenth as much is
absorbed because of the oxalic acid.
Calcium fortification of
everything from orange juice to waffles is
making it easier to get a great deal of calcium
through diet alone. Breakfast cereals have
been fortified for a long time; three-quarters
of a cup of the breakfast cereal whole-grain
Total contains 1,000 mg. For a list of the
calcium content of selected foods, visit our
Web site at www.health.harvard.edu/health.
Supplements
and vitamin pills
Calcium supplements usually
come in 500- to 600-mg tablets. You don’t
absorb large doses of calcium as efficiently
as you do small ones. Thus, much of a 1,000-mg
tablet is going to waste, although unabsorbed
calcium in the gut may have benefits. Taking
a regular-size pill with calcium-fortified
orange juice might also be a waste.
The calcium in most supplements
is either in the form of calcium carbonate
or calcium citrate. Research shows that they
are absorbed equally well with meals, but calcium
carbonate is harder to digest than calcium
citrate. People are usually advised to take
calcium carbonate with or soon after a meal.
Calcium citrate can be taken at any time.
Most multivitamin pills contain
a relatively small amount of calcium — about
100–200 mg.
Problems
with supplements
Acid rebound. Calcium
carbonate may cause acid rebound: the
stomach overcompensates for the high dose of
calcium carbonate, which is alkaline, by churning
out more acid. For that reason, people with
a history of stomach ulcers are advised that
they may not tolerate it and may have to switch
to calcium citrate.
Constipation. Calcium
supplements can have a mild binding effect
but by themselves don’t usually cause
serious constipation. But if you’re taking
another supplement or medication that binds
the stool, the addition of calcium supplements
could cause a problem.
Too much calcium. Although
it doesn’t happen often, some people
have taken so much calcium that it causes hypercalcemia, an
above-normal level of calcium in the blood.
Hypercalcemia may cause nausea, vomiting, confusion,
and other neurological symptoms.
Drug interactions. Large
doses of calcium interfere with the absorption
of a variety of drugs. You should avoid consuming
large amounts of calcium — either in
food or as a supplement — within 2–4
hours of taking a tetracycline or quinolone
antibiotic. After taking alendronate (Fosamax),
risedronate (Actonel), or another one of the
bisphosphonate drugs for osteoporosis, you
should wait at least 30 minutes before consuming
a large amount of calcium. People taking calcium-channel
blockers or beta blockers should discuss taking
calcium with their doctors because it can reduce
the effectiveness of these drugs.
Reading
the labels
The Nutrition Facts
label on food says % DV, which stands
for percent Daily Value. The
Daily Value for a nutrient is set by
FDA regulations. It’s a single
number and not the same as the Recommended
Dietary Allowances (RDAs), which come
from the Institute of Medicine and
often vary with age and sex.
Currently, the Daily
Value for calcium is 1,000 mg — the
RDA for people ages 31 to 50 — not
the 1,200 mg RDA for older adults.
So, obviously, if you see a nutrition
label that says, for example, a serving
contains 20% of the DV, it contains
200 mg of calcium, not 240 mg.
The Supplement Facts
label on supplements and vitamin pills
is also based on the FDA’s Daily
Values, not the RDAs. Although the
calcium in those pills comes in several
forms (calcium carbonate, calcium citrate,
dicalcium phosphate), the amount of
milligrams listed on the label is the
amount of calcium you’re getting,
not the larger compound. |
Other
ways to keep your bones strong
The DOs...
Exercise. Weight-bearing
exercise includes any activity that
pits you against gravity: not just
lifting weights, but walking, climbing
stairs, even dancing. It’s tremendously
important to bone health and preventing
fractures. Your muscles get stronger
and more coordinated, which helps prevent
falls. There’s also a direct
effect on bone. Working muscle stimulates
bone into becoming stronger. If you
want to lower your risk of osteoporosis,
a short, brisk walk every day might
better serve the purpose than a big
calcium pill.
Vitamin D. Getting
enough vitamin D may be the most important
variable in preventing osteoporosis.
Vitamin D’s main function in
the body is to aid calcium absorption.
An analysis of data from the Nurses’ Health
Study found that study participants
who consume 500 IU of vitamin D daily
are 37% less likely to have broken
a hip than women who consume 140 IU.
(IU stands for International Units,
a measure of biological activity.)
Neither total calcium nor milk consumption
was associated with a lower risk for
hip fracture.
The current FDA recommendation
(the Daily Value) is 400 IU daily.
Studies have shown that up to 50% of
older Americans don’t get enough
vitamin D. There are several reasons
for this. The vitamin’s biologically
active form is metabolized when the
skin is exposed to the ultraviolet
radiation in sunlight. Theoretically,
sun exposure can give you all the vitamin
D you need. But north of about 40 degrees
latitude — the latitude of Philadelphia,
Indianapolis, and Denver — the
winter sunlight is too weak to produce
significant amounts of vitamin D. Even
in sunnier climes and times of year,
older people tend to spend a lot of
time indoors. Moreover, older skin
is less effective in making the vitamin
even when it’s exposed to sunlight.
Sunscreens are another problem: they
filter out much of the ultraviolet
radiation that produces vitamin D.
Theoretically, you
could make up for a shortage of sunshine-generated
vitamin D with diet. The problem is
that precious few foods contain the
vitamin. For practical purposes, it’s
limited to several types of saltwater
fish. So decades ago, health officials
in many northern countries decided
to fortify foods with vitamin D. In
the United States, milk — but
not other dairy foods — was chosen.
An 8-ounce glass of milk is supposed
to contain 100 IU, although surveys
have shown that the actual amount can
be a great deal less.
Vitamin K. Your
bones also need vitamin K, which is
found in green, leafy vegetables.
... and the
DON’Ts
Caffeine. If
you’ve got a four-or-more cups-a-day
coffee habit, cut back for the sake
of your bones. High levels of caffeine
may stimulate calcium excretion.
Protein. High
levels of protein, especially animal
protein, alter blood chemistry so calcium
leaches out of your bones. But there
is no cutoff here, just a general word
of caution.
Vitamin A. Several
studies have shown a correlation between
high vitamin A intake and fracture
risk. Some experts say it’s time
to stop fortifying milk and breakfast
cereal with vitamin A, because the
population is getting older and the
prevalence of osteoporosis is increasing.
Meanwhile, you should avoid multivitamins
that contain 10,000 IU of vitamin A,
which is twice the recommended daily
intake of 5,000 IU — and many
experts think that’s too much.
Bear in mind that the beta carotene
in carrots and other vegetables is
not a problem. It is far less biologically
active than retinol, the form
of vitamin A in many vitamin pills
and fortified foods. |
Tips
for increasing your vitamin D intake
Eat more swordfish,
bluefish, salmon, mackerel, or
sardines. But swordfish
and some species of mackerel are
on the list of fish with high mercury
levels, so don’t go overboard.
Take a calcium
supplement that includes vitamin
D. Many brands combine
calcium with 200–400 IU of
vitamin D.
Take a multivitamin. Multivitamins
are a safety net for many nutrients
that might otherwise fall through the
cracks. Most brands contain 400 IU
of vitamin D.
Spend more
time outdoors. This is
a balancing act, because you don’t
want to risk skin cancer in pursuit
of more vitamin D production. |
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